1. Field of the Invention
This invention relates to telephone supervisory signal detection and, more particularly, to a variable threshold detector for dial pulse and dual polarity ring-trip detection.
2. Description of the Prior Art
It is necessary to detect momentary interruptions in a telephone subscriber's loop circuit which represent dial pulses. Such detection must take place at the central office location in order to ascertain the identity of the called number. This function is normally accomplished with a pulsing relay which responds to dial pulses by operating contacts which, in turn, operate dial pulse registration circuitry. Simlarly, it is also necessary to detect the dc current flow in a telephone loop when a subscriber goes off-hook in response to ringing signals.
On exceptionally long subscriber loops, it is often necessary to provide supervisory signaling enhancement at the central office. Such long loops introduce attenuation and distortion into the dial pulse and other supervisory signals which make detection difficult. Special facilities for enhancing supervisory signals as well as providing voice frequency gain are called "range extenders." One such range extender is shown in K. E. Stiefel U.S. Pat. No. 4,056,688, granted Nov. 1, 1977.
The need for range extension on any particular loop is dependent upon the resistance of that loop. Short loops, for example, do not require range extension. In order to provide automatic energization of a range extender for long subscriber loops, it is desirable to detect the resistance of a connected subscriber loop and to switch a range extender into the circuit only for those loops exceeding a predetermined resistance threshold. One such automatic resistance detecting range extender is shown in the copending application of K. E. Stiefel, Ser. No. 85,025, filed of even date herewith.
During dial pulsing, the capacitance of a subscriber loop modifies the waveshape of the dial pulses considerably and renders the accurate detection of dial pulse transitions difficult. Inaccurate dial pulse detection, in turn, can result in the identification of the wrong called subscriber number.
Ring-trip detection, on the other hand, requires the detection of a small direct current flow in the presence of a very large alternating current (the ringing signal). Again, accurate detection is difficult.